Add minimum search length configuration parameter.

Allow users to set their own minimum search term length.  Fixes #2014.

docs/user-guide/configuration.md

@@ -502,6 +502,22 @@ plugins:
 
   **default**: `'[\s\-]+'`
 
+##### **min_search_length**
+
+An integer value that defines the minimum length for a search query. By default
+searches shorter than 3 chars in length are ignored as search result quality with
+short search terms is poor. However, for some use cases (such as documentation
+about Message Queues which might generate searches for 'MQ') it may be preferable
+to set a shorter limit.
+
+```yaml
+plugins:
+    - search:
+        min_search_length: 2
+```
+
+  **default**: 3
+
 ##### **lang**
 
 A list of languages to use when building the search index as identified by their